Silicon nanoparticle white light emitting diode device

ABSTRACT

Multiple films of red-green-blue (RGB) luminescent silicon nanoparticles are integrated in a cascade configuration as a top coating in an ultraviolet/blue light emitting diode (LED) to convert it to a white LED. The configuration of RGB luminescent silicon nanoparticle films harnesses the short wavelength portion of the light emitted from the UV/blue LED while transmitting efficiently the longer wavelength portion. The configuration also reduces damaging heat and/or ultraviolet effects to both the device and to humans.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional PatentApplication Ser. No. 61/021,257 filed on Jan. 15, 2008, which is hereinincorporated by reference.

BACKGROUND

For the past 150 years, lighting technology has been mainly limited toincandescence and fluorescence. While derivative technologies such ashigh-intensity discharge (HID) lamps have emerged, none have achievedenergy efficiencies exceeding 25%, with incandescent lighting achievingan efficiency of less than 2%. With the advent of commercial lightemitting diodes (LEDs) in the 1960s, however, the door was opened for adifferent and exciting form of lighting technology. Unlike conventionallighting, LEDs consume less electricity and have largely avoided theparasitic by-products of its predecessors, namely heat. Early LEDs werered in color, with yellow and orange variants following soon thereafter.To produce white light, however, a blue LED was needed. In 1993, ShujiNakamura of Nichia Chemical Industries produced a blue LED using galliumnitride (GaN). With this development, it was now possible to createwhite light by combining the light of separate LEDs (red, green, andblue), or by creating white LEDs themselves by means of doping.

Solid state lighting (SSL) refers to a type of lighting that utilizesLEDs, organic light-emitting diodes (OLEDs), or polymer light-emittingdiodes (PLEDs) as sources of illumination rather than electricalfilaments or gas. Unlike traditional lighting, SSL creates visible lightwith very little heat or parasitic energy dissipation. Additionally, thesolid-state nature provides for greater resistance to shock, vibration,and wear, thereby increasing lifespan significantly. SSL has beendescribed by the United States Department of Energy as a pivotalemerging technology that promises to alter lighting in the future. It isthe first new lighting technology to emerge in over 40 years and, withits energy efficiencies and cost savings, has the potential to be a verydisruptive technology in the marketplace as well.

A single LED can produce only a limited amount of light, and only asingle color at a time. To produce the white light necessary for SSL,light spanning the visible spectrum (red, green, and blue) must begenerated in correct proportions. To achieve this effect, threeapproaches may be used for generating white light with LEDs: wavelengthconversion, color mixing, and most recently homoepitaxial ZnSe.

Wavelength conversion involves converting some or all of the LED'soutput into visible wavelengths. There are a number of techniques thatmay be used for wavelength conversion. One method is to deposit a yellowphosphor on a blue LED. This is considered an inexpensive method forproducing white light. Blue light produced by an LED excites a phosphor,which then re-emits yellow light. This balanced mixing of yellow andblue lights results in the appearance of white light.

Wavelength conversion may also be accomplished by providing additionalphosphors on a blue LED. This is similar to the process involved withyellow phosphors, except that each excited phosphor re-emits a differentcolor. Similarly, the resulting light is combined with the originatingblue light to create white light. The resulting light, however, has aricher and broader wavelength spectrum and produces a highercolor-quality light, albeit at an increased cost.

Yet another technique to accomplish wavelength conversion is by using anultraviolet (UV) LED coated with doped phosphors which, upon excitation,emit light in the red, green and blue wavelengths. The UV light is usedto excite the different phosphors, which are doped at measured amounts.The colors are mixed resulting in a white light with the richest andbroadest wavelength spectrum.

Another technique for wavelength conversion uses a thin layer ofnanocrystal particles, called quantum dots, containing 33 or 34 pairs ofatoms, primarily cadmium and selenium, which are coated on top of a blueLED. The blue light excites the quantum dots, resulting in a white lightwith a wavelength spectrum similar to UV LEDs.

Color mixing involves utilizing multiple colored LEDs in a lampandadjusting the intensity of each LED to produce white light. For example,the lamp may contain a minimum of two LEDs (blue and yellow), but canalso have three (red, blue, and green) or four (red, blue, green, andyellow). As no phosphors are used, there is no energy lost in theconversion process, thereby exhibiting the potential for higherefficiency. The intensity of the LEDs are configured such that thecombination of the emitted light results in white light.

Wavelength conversion provides benefits versus color mixing. A SSLdevice contains many LEDs placed close together in a lamp to amplifytheir illuminating effects. This is because an individual LED producesonly a limited amount of light, thereby limiting its effectiveness as areplacement light source. In the case where white LEDs are utilized inSSL, this is a relatively simple task, as all LEDs are of the same colorand can be arranged in any fashion. When using the color-mixing method,however, it is more difficult to generate equivalent brightness whencompared to using white LEDs in a similar lamp size. Furthermore,degradation of different LEDs at various times in a color-mixed lamp canlead to an uneven color output. Because of the inherent benefits andgreater number of applications for white LED based SSL, most designsfocus on utilizing them exclusively.

Currently, there is no SSL available that can be offered as a truereplacement for incandescent or fluorescent lamps, even though severalmanufacturers have gone forward with the introduction of such products.White LEDs produced today are too expensive to be considered affordable,and the lumens produced by the LEDs today are not as bright astraditional lighting. Based on research conducted by the United StatesDepartment of Energy (DOE) and the Optoelectronics Industry DevelopmentAssociation (OIDA), it is expected that by the year 2025, SSL will bethe preferred method of illumination in homes and offices.

What is apparent to the end user is the low color rendering index (CRI)of current LEDs. The CRI is widely used to measure how accurately alighting source renders the color of objects. For example, sunlight andincandescent lamps have a CRI of 100, while fluorescent lamps generallyhave a CRI>75. The current generation of LEDs, which employs mostly blueLED chip and yellow phosphor, has a CRI of about 70, which is much toolow for widespread use in lighting, particularly indoor lightingapplications. In order for SSL to effectively replace incandescentlamps, more research must be done on developing alternatives to thetechniques currently used that address these concerns.

There are several advantages to the use of the nano-silicon converter ina white LED. Silicon nanoparticles play a dual role of UV blockers anddown converters of the UV radiation emitted by the LED. Siliconnanoparticles are highly absorbant of the UV with a quantum conversionlarger than 50%. In fact, silicon nanoparticles may act as a total UVfilter, resulting in a safe light source. The silicon nanoparticles staycool because they convert the UV radiation to visible light. The siliconnanoparticles are highly photostable under UV excitation giving a longsafe working lifetime.

Further, a film comprised of silicon nanoparticles acts an excellentantireflection coating preventing light from going back into the LEDhousing causing damage due to heating or direct interaction. The siliconnanoparticle film is transparent in the visible allowing the visiblelight to go through.

The nanoparticles within each color group are identical, allowing theformation of high optical quality films of closely-packed nanoparticles(solid density). This is beneficial because the emission, transmissionand losses of wavelength converter depends sensitively on thicknessuniformity and composition of the converter on the chip.

The nanoparticles can be functionalized (doped) to shift theirluminescence under the same UV source. Producing a Si—C termination onthe particles, for example, shift the spectrum to the silicon carbideemission. This may provide means to improve on filling the whitespectrum to achieve a high CRI ratio in the upper nineties.

SUMMARY

The white light emitting diode of the present disclosure includes anultraviolet/blue light emitting diode (LED) and a converter layerdisposed upon an active region of the ultraviolet/blue light emittingdiode. The converter layer includes a cascade of silicon nanoparticlesconfigured to fluoresce when exposed to light from the ultraviolet/bluelight emitting diode such that the combination of wavelengths of lightemitted from the ultraviolet/blue light emitting diode and emitted byfluorescence of the converter layer produces white light. The converterlayer includes a number of silicon nanoparticle sublayers, wherein eachsublayer is configured to emit fluoresced light in a predeterminedwavelength range of the visible spectrum.

For example, the converter layer may have a first sublayer of siliconnanoparticles configured to emit fluoresced light having a firstwavelength such that the light emitted from the first sublayer is in thered portion of the visible spectrum. The converter layer may also have asecond sublayer of silicon nanoparticles configured to emit fluorescedlight having a second wavelength such that the light emitted from thesecond sublayer is in the green portion of the visible spectrum.Additionally, the converter layer may have a third sublayer of siliconnanoparticles configured to emit fluoresced light having a thirdwavelength such that the light emitted from the third sublayer is in theblue portion of the visible spectrum. The combination of the wavelengthsof the emitted light from the first, second, and third sublayers alongwith the light emitted from the LED producing white light.

The white LED of the present disclosure may also include a dichroic filmlayer located between the UV/blue LED and the converter layer. Thedichroic film allows UV radiation and visible light emitted from theUV/blue LED to pass through while reflecting visible light emitted bythe converter layer away from the LED.

The white LED of the present disclosure may be produced by providing aUV/blue LED and providing a converter layer of silicon nanoparticlesonto an active surface of the UV/blue light emitting diode. Theconverter layer is produced by providing a colloidal suspension ofsilicon nanoparticles in isopropyl alcohol, spreading the colloidalsuspension onto the active surface of the LED, and allowing theisopropyl alcohol to evaporate, resulting in a layer of closely-packednanoparticles. This process may be repeated to produce a number ofsublayers.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will be described hereafter with reference to theattached drawings which are given as a non-limiting example only, inwhich:

FIG. 1 is a schematic cross-sectional representation of the LED of thepresent disclosure; and

FIGS. 2-5 show the down converted spectrum under UV in the range 330-400nm of a variety of silicon nanoparticle populations.

DETAILED DESCRIPTION

The white light emitting diode (LED) 10 of the present disclosureincludes a gallium nitride (GaN) ultraviolet (UV)/blue LED 12 and awavelength converter 14 disposed on an active region of the UV/blue LED,as shown in FIG. 1. The converter layer 14 includes one or morenanoparticle sublayers 16, 18, 20 in a cascade configuration. Thenanoparticles in sublayers 16, 18, and 20 allow blue visible lightemitted by the LED to pass through while absorbing the UV radiationemitted by the LED. The absorbed UV radiation excites the nanoparticleswhich then fluoresce light in wavelengths of the visible spectrum. Thenanoparticle sublayers are configured such that wavelengths offluoresced light combine to produce white light.

In the exemplary embodiment shown, wavelength converter 14 is configuredsuch that each sublayer 16, 18, 20 is tuned to a different section ofthe spectrum by choice of the size of the nanoparticle, namely red 16,green 18, and blue 20 resulting in a red-green-blue (RGB) wavelengthconverter. The wavelength converter 14 is configured in a cascadearrangement to produce red light, which is then transmitted through theblue and green layers; green light, which is transmitted through theblue layer; and blue light; the combination being white light 22.

In the exemplary embodiment of FIG. 1, the wavelength converter 14includes a first sublayer 16 having relatively large siliconnanoparticles tuned to fluoresce light in the red wavelengths of thevisible spectrum. Wavelength converter 14 also includes a secondsublayer 18 having relatively mid-sized silicon nanoparticles tuned tofluoresce light in the green wavelengths of the visible spectrum.Wavelength converter 14 also includes a third sublayer 20 havingrelatively small silicon nanoparticles tuned to fluoresce light in theblue wavelengths of the visible spectrum.

FIGS. 2-5 gives the down converted spectrum under UV in the range330-400 nm of a variety of silicon nanoparticle populations, showingthat it is possible to cover the entire visible spectrum of the solarwhite light (from 400 nm-750 nm) with the device of the presentdisclosure. In addition, the primary blue component from the GaN LED canbe used to further enrich the mixture of emitted light.

The emerging colored light from the sublayers 16, 18, 20 along with someof the remaining LED blue mix together, resulting in a white light withthe richest and broadest wavelength spectrum. The thickness of thesublayers are chosen in conjunction with their characteristicsabsorption/conversion/eye sensitivity to achieve the feel of a sunlightlight source.

The white LED of the present disclosure is produced by starting with agallium nitride (GaN) LED. A colloidal suspension of siliconnanoparticles is prepared in isopropyl alcohol. The active region of theGaN LED is then covered with a layer of silicon nanoparticles byspreading a volume of the particle colloid on the active face. Theisopropyl alcohol is allowed to dry under ambient conditions, resultingin the formation of a thin layer of closely packed particles. Theresponse of the GaN LED is measured before the particle layer is formedand after it has been coated. Additional volume of the colloid is thenplaced on the device and another measurement is taken. This procedure isrepeated several times to allow direct correlation of the response withthe increase in the thickness of the nanoparticle active layer.

The nanoparticles may also be mixed or functionalized with organicpigments to broaden the color composition. The particles may boost theinteraction of UV with the pigment by energy transfer or cascadeexcitation.

The active nanoparticle sublayers not only improve the conversion of UVradiation to visible light but also act as a filter that protects an enduser from the UV radiation emitted from the GaN LED. Also, thenanoparticle film acts as an anti-reflecting coating that stops the UVradiation from reflecting back to the LED, which, if it happens, maycause some damage and shorten the working life of the overall device.Less UV radiation striking back upon the LED device reduces the heatgenerated in the device and hence prolongs the working life.

In addition to the evaporation-based method for deposit of siliconnanoparticles on the GaN LED, other methods such as spin coating, orelectrodeposition may be used. Moreover, alternative methods may be usedto dry the nanoparticle colloidal suspension including mild heating andultraviolet drying, in addition to drying under ambient conditions, aspreviously described herein.

The down conversion spectra of single silicon nanoparticle color samplesin colloids was recorded under irradiation from a 365 nm Hg source. Theconversion efficiency of thin films of single color samples was examinedunder irradiation from a 365 nm Hg source.

Because fluorescence of the silicon nanoparticle sublayers 16, 18, 20 isradiated in all directions equally, half of the response of theparticles to the UV irradiation escapes backward, toward the LED. Thisvisible fluoresced light may be reflected away from the LED, and thusincrease the visible light output of the white LED of the presentdisclosure, by using a dichroic thin film 24. This may be done byplacing an appropriate coating between the nanoparticles of thewavelength converter 14 and the LED 12, as shown in FIG. 1, which allowsUV light to pass through the dichroic film 24 while reflecting thephotoluminescence to the outside, away from the LED. Thus efficiency ofthe LED is further improved by eliminating this loss by redirecting thislight outward.

The foregoing is considered as illustrative only of the principles ofthe claimed invention. Further, since numerous modifications and changeswill readily occur to those skilled in the art, it is not desired tolimit the claimed invention to the exact construction and operationshown and described, and accordingly, all suitable modifications andequivalents may be resorted to, falling within the scope of the claimedinvention.

1. A white light emitting diode comprising: an ultraviolet/blue lightemitting diode, and a converter layer disposed upon an active region ofthe ultraviolet/blue light emitting diode, the converter layercomprising a cascade of silicon nanoparticles configured to fluorescewhen exposed to light from the ultraviolet/blue light emitting diodesuch that the combination of wavelengths of light emitted from theultraviolet/blue light emitting diode and emitted by fluorescence of theconverter layer produces white light.
 2. The white light emitting diodeof claim 1 in which the converter layer comprises a plurality of siliconnanoparticle sublayers, wherein each of the plurality of nanoparticlesublayers is configured to emit fluoresced light in a predeterminedwavelength range of the visible spectrum.
 3. The white light emittingdiode of claim 1 in which the converter layer comprises a sublayer ofsilicon nanoparticles configured to emit fluoresced light having awavelength such that the light emitted from the sublayer is in theyellow portion of the visible spectrum.
 4. The white light emittingdiode of claim 2 in which the converter layer further comprises: a firstsublayer of silicon nanoparticles configured to emit fluoresced lighthaving a first wavelength such that the light emitted from the firstsublayer is in the red portion of the visible spectrum, a secondsublayer of silicon nanoparticles configured to emit fluoresced lighthaving a second wavelength such that the light emitted from the secondsublayer is in the green portion of the visible spectrum, and a thirdsublayer of silicon nanoparticles configured to emit fluoresced lighthaving a third wavelength such that the light emitted from the thirdsublayer is in the blue portion of the visible spectrum.
 5. The whitelight emitting diode of claim 4 in which the converter layer furthercomprises: a fourth sublayer of silicon nanoparticles configured to emitfluoresced light having a fourth wavelength such that the light emittedfrom the fourth sublayer is in the yellow portion of the visiblespectrum.
 6. The white light emitting diode of claim 1 wherein theconverter layer is configured to absorb ultraviolet radiation emittedfrom the ultraviolet/blue light emitting diode.
 7. A method of producinga white light emitting diode comprising the steps of: providing anultraviolet/blue light emitting diode, and providing a converter layerof silicon nanoparticles onto an active surface of the ultraviolet/bluelight emitting diode.
 8. The method of claim 7 wherein theultraviolet/blue light emitting diode is comprised of gallium nitride.9. The method of claim 7 wherein the step of providing a converter layerof silicon nanoparticles onto an active surface of the ultraviolet/bluelight emitting diode further comprises: providing a colloidal suspensionof silicon nanoparticles in isopropyl alcohol, spreading the colloidalsuspension onto an active surface of the light emitting diode, andallowing the isopropyl alcohol to evaporate, resulting in a layer ofclosely-packed nanoparticles.
 10. The method of claim 9 furthercomprising providing an additional layer between the ultraviolet/bluelight emitting diode and the converter layer, the additional layerconfigured to allow ultraviolet light to pass through, away from theultraviolet/blue light emitting diode and the additional layerconfigured to reflect visible light.
 11. The method of claim 10 whereinthe additional layer is a dichroic film.
 12. The method of claim 7wherein the step of providing a converter layer of silicon nanoparticlesonto an active surface of the ultraviolet/blue light emitting diodefurther comprises: providing a first sublayer of silicon nanoparticlesconfigured to emit fluoresced light having a wavelength such that thelight is in the yellow portion of the visible spectrum.
 13. The methodof claim 7 wherein the step of providing a converter layer of siliconnanoparticles onto an active surface of the ultraviolet/blue lightemitting diode further comprises: providing a first sublayer of siliconnanoparticles configured to emit fluoresced light having a wavelengthsuch that the light is in the red portion of the visible spectrum,providing a second sublayer of silicon nanoparticles configured to emitfluoresced light having a second wavelength such that the light is inthe green portion of the visible spectrum, and providing a thirdsublayer of silicon nanoparticles configured to emit fluoresced lighthaving a third wavelength such that the light is in the blue portion ofthe visible spectrum.
 14. The method of claim 13 further comprisingproviding an additional layer between the ultraviolet/blue lightemitting diode and the converter layer, the additional layer configuredto allow ultraviolet light to pass through, away from theultraviolet/blue light emitting diode and the additional layerconfigured to reflect visible light.
 15. The method of claim 14 whereinthe additional layer is a dichroic film.
 16. A white light emittingdiode comprising: a light emitting diode comprised of gallium nitride,and a converter layer comprised of a first sublayer of siliconnanoparticles configured to emit fluoresced light having a firstwavelength such that the light emitted from the first sublayer is in thered portion of the visible spectrum, a second sublayer of siliconnanoparticles configured to emit fluoresced light having a secondwavelength such that the light emitted from the second sublayer is inthe green portion of the visible spectrum, and a third sublayer ofsilicon nanoparticles configured to emit fluoresced light having a thirdwavelength such that the light emitted from the third sublayer is in theblue portion of the visible spectrum wherein the combination of thefluoresced light emitted from the first sublayer, the second sublayerand the third sublayer produces white light.
 17. The white lightemitting diode of claim 16 further comprising a dichroic film betweenthe light emitting diode and the converter layer, wherein the dichroicfilm is configured to allow ultraviolet radiation to pass through awayfrom the light emitting diode, and wherein the dichroic film isconfigured to reflect visible light fluoresced by the converter layeraway from the light emitting diode.